A known developer cartridge is configured to be attached to and detached from an image forming apparatus (e.g., a laser printer) and store toner (e.g., developer) therein. Among various types of image forming apparatuses, an image forming apparatus is configured to determine whether an amount of toner remaining in a developer cartridge is relatively low. Another image forming apparatus is configured to determine whether the number of pages that have been printed in the image forming apparatus is greater than a predetermined number. When a positive determination is made in such a determination in each of the apparatuses, each apparatus controls its display to display thereon information prompting a user to replace a currently-attached developer cartridge with another developing cartridge. In accordance with the information displayed on the display, the user removes the currently-attached developer cartridge and replace with another developer cartridge.